A disk drive system is a digital data storage device that stores information within concentric tracks on a storage disk. The storage disk is coated with a magnetic material that is capable of changing its magnetic orientation in response to an applied magnetic field.
During operation of a disk drive,the disk is rotated about a central axis at a substantially constant rate. To read data from or write data to the disk, a magnetic transducer is positioned above a desired track of the disk while the disk is spinning.
Writing is performed by delivering a write signal having a variable current to the transducer while the transducer is held close to the desired track. The write signal creates a variable magnetic field at a gap portion of the transducer that induces magnetic polarity transitions into the desired track. The magnetic polarity transitions are representative of the data being stored.
Reading is performed by sensing the magnetic polarity transitions on a track with the transducer. As the disk spins below the transducer, the magnetic polarity transitions on the track present a varying magnetic field to the transducer. The transducer converts the varying magnetic field into an analog read signal that is then delivered to a read channel for appropriate processing. The read channel converts the analog read signal into a properly timed digital signal that can be further processed and then provided to a host computer system.
The transducer can include a single element, such as an inductive read/write element for use in both reading and writing, or it can include separate read and write elements. Transducers that include separate elements for reading and writing are known as "dual element heads" and usually include a magneto-resistive (MR) read element for performing the read function.
Dual element heads are advantageous because each element of the transducer can be optimized to perform its particular function. For example, MR read elements are more sensitive to small variable magnetic fields than are inductive heads and, thus, can read much fainter signals from the disk surface. Because MR elements are more sensitive, data can be more densely packed on the surface with no loss of read performance.
MR read elements generally include a strip of magneto-resistive material that is held between two magnetic shields. The resistance of the magneto-resistive material varies almost linearly with the applied magnetic field. During a read operation the MR strip is held near a desired track, specifically, within the varying magnetic field caused by the magnetic transitions on the track. A constant current is passed through the strip resulting in a variable voltage across the strip. By Ohm's law (i.e., V=IR), the variable voltage is proportional to the varying resistance of the MR strip and, hence, is representative of the data stored within the desired track. The variable voltage signal (which is the analog read signal) is then processed and converted to digital form for use by the host.
A standard disk drive, generally designated 10, is illustrated in FIG. 1. The disk drive comprises a disk 12 that is rotated by a spin motor 14. The spin motor 14 is mounted to a base plate 16. An actuator arm assembly 18 is also mounted to the base plate 16.
The actuator arm assembly 18 includes a transducer 20 mounted to a flexure arm 22, which is attached to an actuator arm 24 that can rotate about a bearing assembly 26. The actuator arm assembly 18 includes a voice coil motor (VCM) 28, which moves the transducer 20 relative to the disk 12. The spin motor 14, VCM 28 and transducer 20 are coupled to a number of electronic circuits 30 mounted to a printed circuit board 32. The electronic circuits 30 typically include one or more read channel chips, a microprocessor-based controller and a random access memory (RAM), among other things.
Instead of having a single disk 12 as shown in FIG. 1, as is well-known in the art, the disk drive 10 may include a plurality of disks 12. In such case, each of the plurality of disks 12 would have two sides, with magnetic material on each of those sides. Therefore, two actuator arm assemblies 18 would be provided for each disk 12.
Referring now to FIG. 2, data is stored on the disk 12 within a number of concentric radial tracks 40 (or cylinders). Each track is divided into a plurality of sectors 42. Each sector 42 is further divided into a servo region 44 and a data region 46.
The servo regions 44 of the disk 12 are used to, among other things, accurately position the transducer 20 so that data can be properly written onto and read from the disk 12. The data regions 46 are where non-servo related data (i.e., user data) is stored and retrieved. Such data, upon proper conditions, may be overwritten.
FIG. 3 shows portions of tracks 40 for a disk 12 drawn in a straight, rather than arcuate, fashion for ease of depiction. To accurately write data to and read data from the data region 46 of the disk 12 (see FIG. 2), it is desirable to maintain the transducer 20 in a relatively fixed position with respect to a given track's centerline 48 during each of the writing and reading procedures. Tracks n-1 through n+4, including their corresponding centerlines 48, are shown in FIG. 3.
To assist in controlling the position of the transducer 20 relate to the track centerline 48, the servo region 44 contains, among other things, servo information in the form of servo patterns 50 comprised of one or more groups of servo bursts, as is well-known in the art. First, second, third and fourth servo bursts 52, 54, 56, 58 (commonly referred to as A, B, C and D servo bursts, respectively) are shown in FIG. 3. The servo bursts 52, 54, 56, 58 are accurately positioned relative to the centerline 48 of each track 40. Unike information in the data region 46, servo bursts 52, 54, 56, 58 may not be overwritten or erased during normal operation of the disk drive 10.
As the transducer 20 is positioned over a track 40 (i.e., during a track following procedure), it reads the servo information contained in the servo regions 44 of the track 40, one servo region 44 at a time. The servo information is used to, among other things, generate a position error signal (PES) as a function of the misalignment between the transducer 12 and a desired position relative to the track centerline 48. As is well-known in the art, the PES signals are input to a servo control loop (not shown) which performs calculations and outputs a servo compensation signal which controls the VCM 28 to, ideally, place the transducer 12 at the desired position relative to the track centerline 48.
In addition to performing the track following procedure described above, each track's servo region 44 contains information which is used to position the transducer 20 over an appropriate track 40 and servo region 44 (i.e., to perform seek operations) so that user data may be read from that track's data region 46. More specifically, as shown in FIG. 4, each servo region 44 contains a write/read (W/R) recovery field 60, an automatic gain control (AGC) field 62, a synchronization field 64, a sector number field 66, a cylinder number field 68 and a PES field 70. (The PES field 70 is comprised of servo patterns 50, as described above with reference to FIG. 3).
The W/R field 60 is used by the disk drive 10 to transition from writing data to a previous data region 46 to reading the servo information in the present servo region 44. The AGC field 62 is used to set the gain of the read/write channel (not shown) of the disk drive 10 for optimal performance. The synchronization field 64 is used in synchronizing a system clock so that the sector and cylinder number fields 66, 68 can be read, and so that the PES field 70 can be located.
As is well-known in the art, the sector number field 66 is indicative of the circumferential position of the servo region 44 with respect to the disk 12. Similarly, the cylinder number field 68 includes an address identifying the particular track 40 on which the servo region 44 is located (i.e., the radial position of the servo region 44).
A servo track writer (STW) (not shown) is used to write the servo regions 44, including their corresponding fields, onto the surface(s) of the disk 12 during the manufacturing process. In most present systems, the STW controls write heads (not shown) corresponding to each disk surface of the disk drive system, which write heads are also used to write user data to each disk surface during standard operation of the disk drive. In order to precisely write the servo information onto each surface of the disk 12, the STW directs each write head to write in small steps, with each step having a width (i.e., STW step width 72 as shown in FIG. 5).
FIG. 5 illustrates the relationship between the STW step width 72 and the pitch 74 of the servo region 44 for a conventional system. A for convenience, the tracks 40 are shown as being straight, rather than arcuate, for ease of depiction.
Prior to proceeding further, certain terms should be defined. Specifically, the difference between the terms "pitch" and "width" must be delineated. The term "pitch" is the distance between centers of adjacent regions on the surface of a disk 12. For example, a servo track pitch 74 (shown in the data region 46 of FIG. 5 for convenience) is defined as the distance between the centers of radially adjacent servo regions 44. In contrast, the term "width" is defined as the radial distance from one end to the other end of a single region. For example, a servo track width 75 (shown in the data region 46 of FIG. 5 for convenience) is the width from one end to another of a single servo region 44.
For each servo region 44, the servo track pitch 74 is generally equivalent to the servo track width 75. However, for data regions 46, the data track pitch 76 is generally different from the actual data track width (not shown). Specifically, as will be understood by those skilled in the art, the data track width is generally about 80% of the data track pitch 76 due to the width of the active element of the head being typically less than the data track pitch and the presence of erase bands (not shown), which are typically found on both sides of each data region 46. For simplicity, the effects which reduce the data track width are not shown in the figures. Instead, the data track width is shown to be the same as the data track pitch.
As shown in FIG. 5, for a conventional track 40 (tracks n-2 through n+1 are shown in the figure), the STW must write two steps in order to write one servo region 44, which is equivalent to the width of one track 40, for example, track n. In other words, in conventional systems, the ratio between the STW step width 72 and the pitch 74 of the servo region 44 is 2:1.
In addition, as is well-known to those skilled in the art, the pitch 76 of the data region 46 is approximately equivalent to the pitch 74 of the servo region 44. Accordingly, in conventional systems, the ratio between the number of servo regions 44 to the number of data regions 46 is 1:1.
Relatively recently, there has been a trend to use magneto-resistive (MR) heads instead of thin-film inductive (TFI) heads to perform a disk drive's read functions. One of the main reasons for the switch is due to the greater sensitivity of MR heads over TFI heads.
Conventional MR heads have been designed so that their width is much smaller than the pitch 74 of the servo region 44. Accordingly, with reference to FIGS. 4 and 5, in conventional designs, the width of an MR head is much smaller than the width of the first, second, third and fourth servo bursts 52, 54, 56, 58.
When the MR head width is, for example, about 50% of the pitch 74 of the servo region 44, some relatively minor non-linearities exist in the system with respect to reading servo patterns 50 from the disk 12. However, these minor non-linearities generally do not drastically affect the performance of the disk drive system 10.
When the width of the MR head is reduced to much less than 50%, for example if the MR head width is 37% of the pitch 76 of the data region 46, severe non-linearities can occur. The non-linearities limit the minimum MR head width that can be used for a given data track pitch.
The aforementioned non-linearities may cause the system to improperly position a transducer when reading or writing to or from a disk. Such improper positioning could cause both read and write errors, thus, decreasing the overall performance of the disk drive 10.
Accordingly, there is a need to develop a disk drive system which will reduce the nonlinearities associated with reading servo patterns from a disk, but will still allow the use of narrower MR head read widths.
The present invention is designed to overcome the aforementioned, and other, problems and meet the aforementioned, and other, needs.